Klue, as it happens I am in Covid isolation so have some time!I...

Klue, as it happens I am in Covid isolation so have some time!
I see the downstream as the cherry on top! It requires a niche product, Fancy Yellow or some other specific high value goods, as trying to get into the more mainstream jewellery is over crowded. The tricky bit will be getting noticed as a niche product and having a high enough turnover rate as holding stock is expensive.
Also you don't want to be reliant on other companies production for downstream as you are either competing with others for rough or trying to do an exclusive deal with a producer, may work but hard to find!

Finding your own diamond source is the first, hardest and riskiest! Though can be highly profitable!
Something like 1 in a hundred Kimberlite/Lamproite pipes have diamonds and about 1 in a hundred of those is profitable, though the % increases when diamonds are found in one others nearby more likely to have diamonds.
Usually you would want several options across known diamond provinces, which BDM have.
The search from scratch can take years so BDM's interest in Canada and Ellendale is a potential shortcut.
Once you have found diamonds it is then a case of proving up viability as per Canada and Ellendale/Blina.
In short from here build mine, start processing and sell.

Sales streams are quite reliant on product type, typically a smaller operator will tender goods either monthly or quarterly dependent on volume/value and cashflow, niche goods may be extracted and sold separately or as with BDM downstream.
As I said earlier downstream is heavily reliant on a niche product which is likely available at both sites, Botswana is further away as still early exploration, they are also better known for white diamonds than Fancy colour.
Typically a mine produces less than 0.1% Fancy colour diamonds. Ellendale E9 pipe was around 11% Fancy Yellow and the early Blina production around 7% so likely a blend of E9 and other pipes.

So to sum up the main risk for BDM is not finding profitable mines!
Financing can be difficult.
Cashflow another key.
Bit of a ramble but hope it answers your question.